This invention relates generally to subsurface devices and, more particularly, to improvements in actuators for subsurface devices such as a valve of the type adapted to fail closed in response to the loss of a controlled condition. In one of its important aspects, this invention relates to an actuator of this type for use with a valve configured to control flow through a well bore extending below the earth's surface.
A valve often is installed in the lower end of a well being drilled and in oil or gas well production tubing for closing it in response to the velocity of the flow of well fluid therethrough, and thus, for example, to fail closed in the event of a blowout of the well. In one type of such a valve, the fluid flow velocity to which it reacts by closing, may be changed by adjustment of the sizes of the orifices in the body of the valve through which the well fluid flows or of the force of a spring urging the closure member of the valve to an open state, or both. If, however, in a production well the pressure of the formation from which the fluid is produced drops to a low level and yet the fluid flow velocity remains above the threshold level, adjustment of either or both of the orifice sizes and of the spring force may seriously interfere with production.
In other prior valves of this type, the closure member is adapted to be held open by hydraulic control fluid, which may be supplied thereto from a source position in response to a reduction in the pressure of the control fluid to a predetermined low value. This loss in pressure of the control fluid may in turn occur in response to a predetermined well fluid pressure condition, such as a rapid loss of wellhead flow pressure indicative of a blowout, or to loss of some other controlled condition.
Valves of the latter type require a fluid conduit extending between the wellhead and the fluid responsive operator for the closure member. Thus, their response time is dependent on the distance the fluid conduit must extend, and to maintain a reasonable response time they are ordinarily installed relatively close to the source of control fluid, such as at the mud line of an offshore well. Thus a valve of this type often does not provide protection for the full length of production tubing, as does (for example) a velocity type valve.
These and other problems could be overcome if a valve could be controlled by means at the surface requiring no physical connection with the valve, and thus be installable deep in the well at the valve location, as in the case of storm chokes, but at the same time independently of the condition of the well fluid, as in the case of surface controlled valves. While it has been proposed to communicate with a valve or other subsurface device mechanism deep within a well bore by transmitting electromagnetic signals from the earth's surface (ground or sea level) or from a sea bed, no means has yet been developed to transmit sufficient electromagnetic power through the earth's surface for operating such a valve or subsurface device. While a source of electromagnetic power, such as a battery pack, can be included with the subsurface device the level of electrical power which can be expected to be produced by the same reliably is not sufficient to operate some subsurface devices such as a valve. That is, due to power drain from a battery pack, which is especially rapid in well bores where the temperature may be as high as 300.degree. F., the energy available at the subsurface level would be limited, especially over a long period of time. This would be a particular problem in the case of a subsurface valve of the type described for a production well. Such a valve may be left in a well for years, and may have to operate--i.e., open, close and then reopen--many times, due, for example, to unavoidable losses of the controlled condition or to a planned loss for test purposes.